As a method for producing adiponitrile or its precursor, namely, 1,4-dicyanobutene or 1,4-dicyanobutadiene, by dimerization of acrylonitrile, there have been proposed techniques, such as the one based on an electrolytic dimerization as given in Japanese Patent Kokai No. 27583/1987, the one based on a reaction using a halogenated ruthenium compound and an organic tin halide in the presence of hydrogen as given in Japanese Patent Kokai No. 9531/1994 and the one based on a reaction using a phosphate catalyst and an aromatic amide compound as given in Japanese Patent Kokai No. 1008/1993. The technique based on an electrolytic dimerization has, however, a problem that the energy cost in the production becomes higher and a special installation is required for the production. In the technique based on the reaction using a halogenated ruthenium compound or a phosphite catalyst, problems undesirable for an industrial production are brought about in, for example, that the reaction velocity is low, that the selectivity of the straight chain dimer is low and that the life of the catalyst is short.
Therefore, the object of the present invention is to provide an industrially acceptable method capable of producing a straight chain dimer of acrylonitrile efficiently at a high yield in a simple manner using a highly active catalyst exhibiting a superior stability, without suffering from occurrence of difficultly removable by-products.